awesome/lib/awful/spawn.lua

--luacheck: no max line length

---------------------------------------------------------------------------
--- Spawning of programs.
--
-- This module provides methods to start programs and supports startup
-- notifications, which allows for callbacks and applying properties to the
-- program after it has been launched.  This requires currently that the
-- applicaton supports them.
--
-- Frequently asked questions
-- ===
--
-- **Rules of thumb when a shell is needed**:
--
-- * A shell is required when the commands contain `&&`, `;`, `||`, `&` or
--  any other unix shell language syntax
-- * When shell variables are defined as part of the command
-- * When the command is a shell alias
--
-- Note that a shell is **not** a terminal emulator. A terminal emulator is
-- something like XTerm, Gnome-terminal or Konsole. A shell is something like
-- `bash`, `zsh`, `busybox sh` or `Debian ash`.
--
-- If you wish to open a process in a terminal window, check that your terminal
-- emulator supports the common `-e` option. If it does, then something like
-- this should work:
--
--    awful.spawn(terminal.." -e my_command")
--
-- Note that some terminals, such as rxvt-unicode (urxvt) support full commands
-- using quotes, while other terminal emulators require to use quoting.
--
-- **Understanding clients versus PID versus commands versus class**:
--
-- A *process* has a *PID* (process identifier). It can have 0, 1 or many
-- *window*s.
--
-- A *command* is what is used to start *process*(es). It has no direct relation
-- with *process*, *client* or *window*. When a command is executed, it will
-- usually start a *process* which keeps running until it exits. This however is
-- not always the case as some applications use scripts as command and others
-- use various single-instance mechanisms (usually client/server) and merge
-- with an existing process.
--
-- A *client* corresponds to a *window*. It is owned by a process. It can have
-- both a parent and one or many children. A *client* has a *class*, an
-- *instance*, a *role*, and a *type*. See `client.class`, `client.instance`,
-- `client.role` and `client.type` for more information about these properties.
--
-- **Understanding blocking versus asynchronous execution**:
--
-- Awesome is single threaded, it means only one thing is executed at any time.
-- But Awesome isn't doomed to be slow. It may not have multiple threads, but
-- it has something called coroutine and also has callbacks. This means things
-- that take time to execute can still do so in the background (using C threads
-- or external process + sockets). When they are done, they can notify the
-- Awesome thread. This works perfectly and avoid blocking Awesome.
--
-- If you want to update the text of a `wibox.widget.textbox` with the output
-- of a shell command, you should use the `awful.spawn.easy_async_with_shell`
-- command. It is strongly recommanded not to use `io.popen` is explained in the
-- "Getting a command's output" section. Asynchronous execution is at first a
-- bit tricky to understand if you never used that before. The example below
-- should demonstrate how it works.
--
-- If we do (but *really*, don't do that):
--
--     -- **NEVER, EVER, DO THIS**: your computer will freeze
--     os.execute("sleep 1; echo foo > /tmp/foo.txt")
--     mylabel.text = io.popen("cat /tmp/foo.txt"):read("*all")
--
-- The label will display `foo`. But If we do:
--
--     -- Don't do this, it wont work.
--     -- Assumes /tmp/foo.txt does not exist
--     awful.spawn.with_shell("sleep 1; echo foo > /tmp/foo.txt")
--     mylabel.text = io.popen("cat /tmp/foo.txt"):read("*all")
--
-- Then the label will be **empty**. `awful.spawn` and `awful.spawn.with_shell`
-- will **not** block and thus the `io.popen` will be executed before
-- `sleep 1` finishes. This is why we have async functions to execute shell
-- commands. There are many variants with difference characteristics and
-- complexity. `awful.spawn.easy_async` is the most common as it is good enough
-- for the general "I want to execute a command and do something with the
-- output when it finishes".
--
--     -- This is the correct way
--     local command = "sleep 1; echo foo > /tmp/foo.txt"
--
--     awful.spawn.easy_async_with_shell(command, function()
--         awful.spawn.easy_async_with_shell("cat /tmp/foo.txt", function(out)
--             mylabel.text = out
--         end)
--     end)
--
-- In this variant, Awesome will not block. Again, like other spawn, you
-- cannot add code outside of the callback function to use the result of the
-- command. The code will be executed before the command is finished so the
-- result wont yet be available.
--
-- **Getting a command's output**:
--
-- First, do **not** use `io.popen` **ever**. It is synchronous. Synchronous
-- functions **block everything** until they are done. All visual applications
-- lock (as Awesome no longer responds), you will probably lose some keyboard
-- and mouse events and will have higher latency when playing games. This is
-- also true when reading files synchronously, but this is another topic.
--
-- Awesome provides a few ways to get output from commands. One is to use the
-- `Gio` libraries directly. This is usually very complicated, but gives a lot
-- of control on the command execution.
--
-- This modules provides `with_line_callback` and `easy_async` for convenience.
-- First, lets add this bash command to `rc.lua`:
--
--    local noisy = [[bash -c '
--      for I in $(seq 1 5); do
--        date
--        echo err >&2
--        sleep 2
--      done
--    ']]
--
-- It prints a bunch of junk on the standard output (*stdout*) and error
-- (*stderr*) streams. This command would block Awesome for 10 seconds if it
-- were executed synchronously, but will not block it at all using the
-- asynchronous functions.
--
-- `with_line_callback` will execute the callbacks every time a new line is
-- printed by the command:
--
--    awful.spawn.with_line_callback(noisy, {
--        stdout = function(line)
--            naughty.notification { message = "LINE:"..line }
--        end,
--        stderr = function(line)
--            naughty.notification { message = "ERR:"..line}
--        end,
--    })
--
-- If only the full output is needed, then `easy_async` is the right choice:
--
--    awful.spawn.easy_async(noisy, function(stdout, stderr, reason, exit_code)
--        naughty.notification { message = stdout }
--    end)
--
-- **Default applications**:
--
-- If the intent is to open a file/document, then it is recommended to use the
-- following standard command. The default application will be selected
-- according to the [Shared MIME-info Database](https://specifications.freedesktop.org/shared-mime-info-spec/shared-mime-info-spec-latest.html)
-- specification. The `xdg-utils` package provided by most distributions
-- includes the `xdg-open` command:
--
--    awful.spawn({"xdg-open", "/path/to/file"})
--
-- Awesome **does not** manage, modify or otherwise influence the database
-- for default applications. For information about how to do this, consult the
-- [ARCH Linux Wiki](https://wiki.archlinux.org/index.php/default_applications).
--
-- If you wish to change how the default applications behave, then consult the
-- [Desktop Entry](https://specifications.freedesktop.org/desktop-entry-spec/desktop-entry-spec-latest.html)
-- specification.
--
-- Spawning applications with specific properties
-- ===
--
-- **The startup notification protocol**:
--
-- The startup notification protocol is an optional specification implemented
-- by X11 applications to bridge the chain of knowledge between the moment a
-- program is launched to the moment its window (client) is shown. It can be
-- found [on the FreeDesktop.org website](https://www.freedesktop.org/wiki/Specifications/startup-notification-spec/).
--
-- Awesome has support for the various events that are part of the protocol, but
-- the most useful is the identifier, usually identified by its `SNID` acronym in
-- the documentation. It isn't usually necessary to even know it exists, as it
-- is all done automatically. However, if more control is required, the
-- identifier can be specified by an environment variable called
-- `DESKTOP_STARTUP_ID`. For example, let us consider execution of the following
-- command:
--
--    DESKTOP_STARTUP_ID="something_TIME$(date '+%s')" my_command
--
-- This should (if the program correctly implements the protocol) result in
-- `c.startup_id` to at least match `something`.
-- This identifier can then be used in `ruled.client` to configure the client.
--
-- Awesome can automatically set the `DESKTOP_STARTUP_ID` variable. This is used
-- by `awful.spawn` to specify additional rules for the startup. For example:
--
--    awful.spawn("urxvt -e maxima -name CALCULATOR", {
--        floating  = true,
--        tag       = mouse.screen.selected_tag,
--        placement = awful.placement.bottom_right,
--    })
--
-- This can also be used from the command line:
--
--    awesome-client 'awful=require("awful");
--      awful.spawn("urxvt -e maxima -name CALCULATOR", {
--        floating  = true,
--        tag       = mouse.screen.selected_tag,
--        placement = awful.placement.bottom_right,
--      })'
--
-- This table contains the client properties that are valid when used the
-- `sn_rules` or `prop` function argument. They are the same as in `ruled.client`.
--
--@DOC_rules_index_COMMON@
--
-- @author Julien Danjou <julien@danjou.info>
-- @author Emmanuel Lepage Vallee <elv1313@gmail.com>
-- @copyright 2008 Julien Danjou
-- @copyright 2014 Emmanuel Lepage Vallee
-- @module awful.spawn
---------------------------------------------------------------------------

local capi =
{
    awesome = awesome,
    mouse = mouse,
    client = client,
}
local lgi = require("lgi")
local Gio = lgi.Gio
local GLib = lgi.GLib
local util   = require("awful.util")
local gtable = require("gears.table")
local gtimer = require("gears.timer")
local aclient = require("awful.client")
local protected_call = require("gears.protected_call")

local spawn = {}


local end_of_file
do
    -- API changes, bug fixes and lots of fun. Figure out how a EOF is signalled.
    local input
    if not pcall(function()
        -- No idea when this API changed, but some versions expect a string,
        -- others a table with some special(?) entries
        input = Gio.DataInputStream.new(Gio.MemoryInputStream.new_from_data(""))
    end) then
        input = Gio.DataInputStream.new(Gio.MemoryInputStream.new_from_data({}))
    end
    local line, length = input:read_line()
    if not line then
        -- Fixed in 2016: NULL on the C side is transformed to nil in Lua
        end_of_file = function(arg)
            return not arg
        end
    elseif tostring(line) == "" and #line ~= length then
        -- "Historic" behaviour for end-of-file:
        -- - NULL is turned into an empty string
        -- - The length variable is not initialized
        -- It's highly unlikely that the uninitialized variable has value zero.
        -- Use this hack to detect EOF.
        end_of_file = function(arg1, arg2)
            return #arg1 ~= arg2
        end
    else
        assert(tostring(line) == "", "Cannot determine how to detect EOF")
        -- The above uninitialized variable was fixed and thus length is
        -- always 0 when line is NULL in C. We cannot tell apart an empty line and
        -- EOF in this case.
        require("gears.debug").print_warning("Cannot reliably detect EOF on an "
                .. "GIOInputStream with this LGI version")
        end_of_file = function(arg)
            return tostring(arg) == ""
        end
    end
end

local function hash_command(cmd, rules)
    rules = rules or {}
    cmd = type(cmd) == "string" and cmd or table.concat(cmd, ';')

    -- Do its best at adding some entropy
    local concat_rules = nil
    concat_rules = function (r, depth)
        if depth > 2 then return end

        local keys = gtable.keys(rules)

        for _, k in ipairs(keys) do
            local v = r[k]
            local t = type(v)

            if t == "string" or t == "number" then
                cmd = cmd..k..v
            elseif t == "tag" then
                cmd = cmd..k..v.name
            elseif t == "table" and not t.connect_signal then
                cmd = cmd .. k
                concat_rules(v, depth + 1)
            end
        end
    end

    concat_rules(rules, 1)

    local glibstr = GLib.String(cmd)

    return string.format('%x', math.ceil(GLib.String.hash(glibstr)))
end

spawn.snid_buffer = {}

function spawn.on_snid_callback(c)
    local entry = spawn.snid_buffer[c.startup_id]
    if entry then
        local props = entry[1]
        local callback = entry[2]
        --TODO v5: Remove this signal
        c:emit_signal("spawn::completed_with_payload", props, callback)

        gtimer.delayed_call(function()
            spawn.snid_buffer[c.startup_id] = nil
        end)
    end
end

function spawn.on_snid_cancel(id)
    if spawn.snid_buffer[id] then
        spawn.snid_buffer[id] = nil
    end
end

--- Spawn a program, and optionally apply properties and/or run a callback.
--
-- Applying properties or running a callback requires the program/client to
-- support startup notifications.
--
-- See `ruled.client.execute` for more details about the format of `sn_rules`.
--
-- @tparam string|table cmd The command.
-- @tparam[opt=true] table|boolean sn_rules A table of properties to be applied
--   after startup; `false` to disable startup notifications.
-- @tparam[opt] function callback A callback function to be run after startup.
-- @treturn[1] integer The forked PID.
-- @treturn[1] ?string The startup notification ID, if `sn` is not false, or
--   a `callback` is provided.
-- @treturn[2] string Error message.
-- @staticfct awful.spawn
function spawn.spawn(cmd, sn_rules, callback)
    if cmd and cmd ~= "" then
        local enable_sn = (sn_rules ~= false or callback)
        enable_sn = not not enable_sn -- Force into a boolean.
        local pid, snid = capi.awesome.spawn(cmd, enable_sn)
        -- The snid will be nil in case of failure
        if snid then
            sn_rules = type(sn_rules) ~= "boolean" and sn_rules or {}
            spawn.snid_buffer[snid] = { sn_rules, { callback } }
        end
        return pid, snid
    end
    -- For consistency
    return "Error: No command to execute"
end

--- Spawn a program using the shell.
-- This calls `cmd` with `$SHELL -c` (via `awful.util.shell`).
-- @tparam string cmd The command.
-- @staticfct awful.spawn.with_shell
function spawn.with_shell(cmd)
    if cmd and cmd ~= "" then
        cmd = { util.shell, "-c", cmd }
        return capi.awesome.spawn(cmd, false)
    end
end

--- Spawn a program and asynchronously capture its output line by line.
-- @tparam string|table cmd The command.
-- @tab callbacks Table containing callbacks that should be invoked on
--   various conditions.
-- @tparam[opt] function callbacks.stdout Function that is called with each
--   line of output on stdout, e.g. `stdout(line)`.
-- @tparam[opt] function callbacks.stderr Function that is called with each
--   line of output on stderr, e.g. `stderr(line)`.
-- @tparam[opt] function callbacks.output_done Function to call when no more
--   output is produced.
-- @tparam[opt] function callbacks.exit Function to call when the spawned
--   process exits. This function gets the exit reason and code as its
--   arguments.
--   The reason can be "exit" or "signal".
--   For "exit", the second argument is the exit code.
--   For "signal", the second argument is the signal causing process
--   termination.
-- @treturn[1] Integer the PID of the forked process.
-- @treturn[2] string Error message.
-- @staticfct awful.spawn.with_line_callback
function spawn.with_line_callback(cmd, callbacks)
    local stdout_callback, stderr_callback, done_callback, exit_callback =
        callbacks.stdout, callbacks.stderr, callbacks.output_done, callbacks.exit
    local have_stdout, have_stderr = stdout_callback ~= nil, stderr_callback ~= nil
    local pid, _, stdin, stdout, stderr = capi.awesome.spawn(cmd,
            false, false, have_stdout, have_stderr, exit_callback)
    if type(pid) == "string" then
        -- Error
        return pid
    end

    local done_before = false
    local function step_done()
        if have_stdout and have_stderr and not done_before then
            done_before = true
            return
        end
        if done_callback then
            done_callback()
        end
    end
    if have_stdout then
        spawn.read_lines(Gio.UnixInputStream.new(stdout, true),
                stdout_callback, step_done, true)
    end
    if have_stderr then
        spawn.read_lines(Gio.UnixInputStream.new(stderr, true),
                stderr_callback, step_done, true)
    end
    assert(stdin == nil)
    return pid
end

--- Asynchronously spawn a program and capture its output.
-- (wraps `spawn.with_line_callback`).
-- @tparam string|table cmd The command.
-- @tab callback Function with the following arguments
--   @tparam string callback.stdout Output on stdout.
--   @tparam string callback.stderr Output on stderr.
--   @tparam string callback.exitreason Exit reason ("exit" or "signal").
--   @tparam integer callback.exitcode Exit code (exit code or signal number,
--     depending on "exitreason").
-- @treturn[1] Integer the PID of the forked process.
-- @treturn[2] string Error message.
-- @see spawn.with_line_callback
-- @staticfct awful.spawn.easy_async
function spawn.easy_async(cmd, callback)
    local stdout = ''
    local stderr = ''
    local exitcode, exitreason
    local function parse_stdout(str)
        stdout = stdout .. str .. "\n"
    end
    local function parse_stderr(str)
        stderr = stderr .. str .. "\n"
    end
    local function done_callback()
        return callback(stdout, stderr, exitreason, exitcode)
    end
    local exit_callback_fired = false
    local output_done_callback_fired = false
    local function exit_callback(reason, code)
        exitcode = code
        exitreason = reason
        exit_callback_fired = true
        if output_done_callback_fired then
            return done_callback()
        end
    end
    local function output_done_callback()
        output_done_callback_fired = true
        if exit_callback_fired then
            return done_callback()
        end
    end
    return spawn.with_line_callback(
        cmd, {
        stdout=parse_stdout,
        stderr=parse_stderr,
        exit=exit_callback,
        output_done=output_done_callback
    })
end

--- Call `spawn.easy_async` with a shell.
-- This calls `cmd` with `$SHELL -c` (via `awful.util.shell`).
-- @tparam string cmd The command.
-- @tab callback Function with the following arguments
--   @tparam string callback.stdout Output on stdout.
--   @tparam string callback.stderr Output on stderr.
--   @tparam string callback.exitreason Exit reason ("exit" or "signal").
--   @tparam integer callback.exitcode Exit code (exit code or signal number,
--     depending on "exitreason").
-- @treturn[1] Integer the PID of the forked process.
-- @treturn[2] string Error message.
-- @see spawn.with_line_callback
-- @staticfct awful.spawn.easy_async_with_shell
function spawn.easy_async_with_shell(cmd, callback)
    return spawn.easy_async({ util.shell, "-c", cmd or "" }, callback)
end

--- Read lines from a Gio input stream
-- @tparam Gio.InputStream input_stream The input stream to read from.
-- @tparam function line_callback Function that is called with each line
--   read, e.g. `line_callback(line_from_stream)`.
-- @tparam[opt] function done_callback Function that is called when the
--   operation finishes (e.g. due to end of file).
-- @tparam[opt=false] boolean close Should the stream be closed after end-of-file?
-- @staticfct awful.spawn.read_lines
function spawn.read_lines(input_stream, line_callback, done_callback, close)
    local stream = Gio.DataInputStream.new(input_stream)
    local function done()
        if close then
            stream:close()
        end
        stream:set_buffer_size(0)
        if done_callback then
            protected_call(done_callback)
        end
    end
    local start_read, finish_read
    start_read = function()
        stream:read_line_async(GLib.PRIORITY_DEFAULT, nil, finish_read)
    end
    finish_read = function(obj, res)
        local line, length = obj:read_line_finish(res)
        if type(length) ~= "number" then
            -- Error
            print("Error in awful.spawn.read_lines:", tostring(length))
            done()
        elseif end_of_file(line, length) then
            -- End of file
            done()
        else
            -- Read a line
            -- This needs tostring() for older lgi versions which returned
            -- "GLib.Bytes" instead of Lua strings (I guess)
            protected_call(line_callback, tostring(line))

            -- Read the next line
            start_read()
        end
    end
    start_read()
end

-- When a command should only be executed once or only have a single instance,
-- track the SNID set on them to prevent multiple execution.
spawn.single_instance_manager = {
    by_snid = {},
    by_pid  = {},
    by_uid  = {},
}

aclient.property.persist("single_instance_id", "string")

-- Check if the client is running either using the rule source or the matcher.
local function is_running(hash, matcher)
    local status = spawn.single_instance_manager.by_uid[hash]
    if not status then return false end

    if matcher then
        for _, c in ipairs(client.get()) do
            if matcher(c) then return true end
        end
    end

    if #status.instances == 0 then return false end

    for _, c in ipairs(status.instances) do
        if c.valid then return true end
    end

    return false
end

-- Keep the data related to this hash.
local function register_common(hash, rules, matcher, callback)
    local status = spawn.single_instance_manager.by_uid[hash]
    if status then return status end

    status = {
        rules     = rules,
        callback  = callback,
        instances = setmetatable({}, {__mode = "v"}),
        hash      = hash,
        exec      = false,
        matcher   = matcher,
    }

    spawn.single_instance_manager.by_uid[hash] = status

    return status
end

local function run_once_common(hash, cmd, keep_pid)
    local pid, snid = spawn.spawn(cmd)

    if type(pid) == "string" or not snid then return pid, snid end

    assert(spawn.single_instance_manager.by_uid[hash])

    local status = spawn.single_instance_manager.by_uid[hash]
    status.exec = true

    spawn.single_instance_manager.by_snid[snid] = status

    if keep_pid then
        spawn.single_instance_manager.by_pid[pid] = status
    end

    -- Prevent issues related to PID being re-used and a memory leak
    gtimer {
        single_shot = true,
        autostart   = true,
        timeout     = 30,
        callback    = function()
            spawn.single_instance_manager.by_pid [pid ] = nil
            spawn.single_instance_manager.by_snid[snid] = nil
        end
    }

    return pid, snid
end

local function run_after_startup(f)
    -- The clients are not yet managed during the first execution, so it will
    -- miss existing instances.
    if awesome.startup then
        gtimer.delayed_call(f)
    else
        f()
    end
end

--- Spawn a command if it has not been spawned before.
--
-- This function tries its best to preserve the state across `awesome.restart()`.
--
-- By default, when no `unique_id` is specified, this function will generate one by
-- hashing the command and its rules. If you have multiple instance of the same
-- command and rules, you need to specify an UID or only the first one will be
-- executed.
--
-- The `rules` are standard `ruled.client`.
--
-- This function depends on the startup notification protocol to be correctly
-- implemented by the command. See `client.startup_id` for more information.
-- Note that this also wont work with shell or terminal commands.
--
-- @tparam string|table cmd The command.
-- @tparam[opt] table rules The properties that need to be applied to the client.
-- @tparam[opt] function matcher A matching function to find the instance
--  among running clients.
-- @tparam[opt] string unique_id A string to identify the client so it isn't executed
--  multiple time.
-- @tparam[opt] function callback A callback function when the client is created.
-- @see ruled.client
-- @staticfct awful.spawn.once
function spawn.once(cmd, rules, matcher, unique_id, callback)
    local hash = unique_id or hash_command(cmd, rules)
    local status = register_common(hash, rules, matcher, callback)
    run_after_startup(function()
        if not status.exec and not is_running(hash, matcher) then
            run_once_common(hash, cmd, matcher ~= nil)
        end
    end)
end

--- Spawn a command if an instance is not already running.
--
-- This is like `awful.spawn.once`, but will spawn new instances if the previous
-- has finished.
--
-- The `rules` are standard `ruled.client`.
--
-- This function depends on the startup notification protocol to be correctly
-- implemented by the command. See `client.startup_id` for more information.
-- Note that this also wont work with shell or terminal commands.
--
-- Note that multiple instances can still be spawned if the command is called
-- faster than the client has time to start.
--
-- @tparam string|table cmd The command.
-- @tparam[opt] table rules The properties that need to be applied to the client.
-- @tparam[opt] function matcher A matching function to find the instance
--  among running clients.
-- @tparam[opt] string unique_id A string to identify the client so it isn't executed
--  multiple time.
-- @tparam[opt] function callback A callback function when the client is created.
-- @see ruled.client
-- @staticfct awful.spawn.single_instance
function spawn.single_instance(cmd, rules, matcher, unique_id, callback)
    local hash = unique_id or hash_command(cmd, rules)
    register_common(hash, rules, matcher, callback)
    run_after_startup(function()
        if not is_running(hash, matcher) then
            return run_once_common(hash, cmd, matcher ~= nil)
        end
    end)
end

local raise_rules = {focus = true, switch_to_tags = true, raise = true}

--- Raise a client if it exists or spawn a new one then raise it.
--
-- This function depends on the startup notification protocol to be correctly
-- implemented by the command. See `client.startup_id` for more information.
-- Note that this also wont work with shell or terminal commands.
--
-- @tparam string|table cmd The command.
-- @tparam table rules The properties that need to be applied to the client.
-- @tparam[opt] function matcher A matching function to find the instance
--  among running clients.
-- @tparam[opt] string unique_id A string to identify the client so it isn't executed
--  multiple time.
-- @tparam[opt] function callback A callback function when the client is created.
-- @see ruled.client
-- @treturn client The client if it already exists.
-- @staticfct awful.spawn.raise_or_spawn
-- @request client activate spawn.raise_or_spawn granted Activate a client when
--  `awful.spawn.raise_or_spawn` is called and the client exists.
function spawn.raise_or_spawn(cmd, rules, matcher, unique_id, callback)
    local hash = unique_id or hash_command(cmd, rules)

    local status = spawn.single_instance_manager.by_uid[hash]
    if status then
        for _, c in ipairs(status.instances) do
            if c.valid then
                c:emit_signal("request::activate", "spawn.raise_or_spawn", raise_rules)
                return c
            end
        end
    end

    -- Do not modify the original. It also can't be a metatable.__index due to
    -- its "broken" `pairs()` support.
    local props = gtable.join(rules, raise_rules)

    spawn.single_instance(cmd, props, matcher, unique_id, callback)

    return nil
end

capi.awesome.connect_signal("spawn::canceled" , spawn.on_snid_cancel   )
capi.awesome.connect_signal("spawn::timeout"  , spawn.on_snid_cancel   )
capi.client.connect_signal ("request::manage" , spawn.on_snid_callback )

return setmetatable(spawn, { __call = function(_, ...) return spawn.spawn(...) end })
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